Relating to the related art following discussions are presented.
In a conventional circuit for driving a laser diode, a transistor serving as the current source of a differential transistor pair is controlled by the output voltage of an APC circuit (automatic power control circuit), whereby the pulsed driving current of the laser diode is adjusted in dependence upon a change in ambient temperature. FIG. 2 is a diagram illustrating an example of the construction of such a laser-diode drive circuit.
As shown in FIG. 2, the conventional laser-diode drive circuit includes a differential transistor pair 1 comprising a pair of transistors whose emitter electrodes are connected together, emitter-follower transistors 2 and 3 for supplying a base voltage to each of the transistors of the differential transistor pair 1, a transistor 4 for supplying a bias current to a laser diode 13, a transistor 5 serving as the current source of the differential transistor pair 1, emitter resistors 10, 11, 8 and 9 which are connected to transistors 2, 3, 4 and 5, respectively, and to a low-potential side of a power supply, an APC circuit 12, and the laser diode 13.
The collector electrode of one transistor of the differential transistor pair 1 and the collector electrode of the transistor 4 which supplies the bias current to the laser diode are both connected to the cathode electrode of the laser diode 13, the collector electrode of the current-source transistor 5 is connected to the common emitter electrodes of the differential transistor pair 1, and the base electrodes of the transistors 4 and 5 are-connected to the control output side of the APC circuit 12.